Where did it all go??

Cole met with his nutritionist eight weeks ago.  At that point, he was 18 pounds and a half ounce.  Since then, although his recommended intake is 900 calories, he has been consuming anywhere from 900-1500 calories depending on the day.  He eats the highest fat foods I can find and make, including ice cream smoothies 1 to 2 times a day.  This week, we met with her again and Cole was 18 pounds and a half ounce.  Needless to say, the boy is once again off the charts.   However, with his growth of an inch and a half in length, in addition to his diet and intake, she was not concerned and just recommended continuing to focus on calorie dense foods.  That's really no problem, but I really would like to know how I can eat a macadamia nut ice cream smoothie everyday and not gain an ounce!